Direct fired furnace

ABSTRACT

A direct fired furnace having two or more confluent chambers with means for segregating the atmosphere in the respective chambers. Each chamber has high velocity burners placed above the work to create a strong convection pattern. A gas passageway is located beneath the work with a central flue located intermediate the chambers, which central flue may lead to an incinerator.

Unite States atet r191 Hemsath et al.

[ Jan. 14, 1975 4] DIRECT FIRED FURNACE [75] Inventors: Klaus H.Hemsath; Harry Dean Schmidt, both of Sylvania; Arvind C. Thekdi, Toledo,all of Ohio [73] Assignee: Midland-Ross Corporation,

Cleveland, Ohio [22] Filed: May 2, 1973 [21] Appl. No.: 356,402

[52] US. Cl. 266/5, 432/128 [51] Int. Cl C2ld 1/08 [58] Field of Search266/5 R, 5 F, 5 T;

[56] References Cited UNITED STATES PATENTS 2,886,303 5/1959 Rusciano266/5 3,637,198 1/1972 Knaak 432/128 Primary ExaminerRoy Lake AssistantExaminer-DeWalden W. Jones Attorney, Agent, or FirmPeter Vrahotes; FrankJ. Nawalanic [57] ABSTRACT A direct fired furnace having two or moreconfluent chambers with means for segregating the atmosphere in therespective chambers. Each chamber has high velocity burners placed abovethe work to create a strong convection pattern. A gas passageway islocated beneath the work with a central flue located intermediate thechambers, which central flue may lead to an incinerator.

7 Claims, 2 Drawing Figures DIRECT FIRED FURNACE BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to directlyheated furnaces and, more specifically, to furnaces wherein confluentchambers are present which require a segregation of atmospheres. Anexample of such a furnace is one which is used to dewax and cure metalcasting molds.

2. Description of Prior Art In certain furnace operations, it isdesirable to exercise atmosphere control over adjacent and confluentzones. As an example of such furnaces, in U.S. Pat. No. 3,189,336,issued to .I. Montagino, a furnace is shown wherein a double archconstruction yields a buffer zone that results in segregation ofadjacent confluent zones. Also, in U.S. Pat. No. Re. 26,935, issued toO. E. Cullen, a furnace is shown wherein zone control is exercised againthrough use of double arches and selected flue locations. Neither ofthese furnaces, however, is a directly heated furnace, each one beingheated by radiant tubes. Additionally, each of these furnaces requiresthe use of fans for the purpose of circulating the atmosphere withineach of their treating zones. To the applicants knowledge, no directfired furnace has heretofore successfully accomplished the segregationof furnace zones. A particular application in which such a directlyheated furnace having zone control is applicable is in the field ofdewaxing and curing of metal casting molds. Prior art furnaces wouldburn the wax in a first zone and pass the work into a second zonewherein the work would then be cured, there being an interchange ofatmosphere between such zones. There were two disadvantages to thisprior art method in that the burned waxes would accompany the work intothe second chamber, thereby interfering with the results achieved, andthe off-gases would be extremely smokey, a condition that is no longerpermissible under antipollution regulations.

SUMMARY OF THE INVENTION This invention is demonstrated by itsapplication with regard to a dewaxing furnace, but it will be understoodthat the invention has application with other types of furnaces as well.A standard method for fabricating metal castings is to make a wax modelof the part to be cast, then dip the wax model into a slurry of ceramicmaterial. After the slurry is cured, the wax model is eliminated in adewaxing furnace. The dewaxing furnace illustrated herein has twochambers, a low temperature chamber and a second or high temperaturechamber. There is zone separation without internal doors through the useof a drop arch and burner and flue placement. Proper operation of thefurnace results in atmosphere control to eliminate uncontrolled burningof wax within the furnace. Instead, the wax is melted and vaporized, asopposed to being melted and burned, and the wax is removed from thefurnace without entering into the second zone. After the wax is removed,the molds are conveyed into the second, or higher temperature, zonewherein the heat treating and curing takes place. Circulation of theatmosphere within each of the zones is accomplished by burners whichexercise a high degree of entrainment. The burners are located above thework and a flue system is located below the work, thereby creatingcirculation of the atmosphere within each zone which yields uniform gastemperatures. The circulating flow pattern thus achieved results inuniform heat treatment of the work. Obviously, another advantage tohaving segregated zones is that greater temperature control is achievedby preventing the gas in the first zone from entering the second zone.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERREDEMBODIMENTS Referring now to the drawing, a direct fired heat treatingfurnace is shown generally at 10 and has a housing comprising arefractory roof l2, refractory side walls .14, front and back end wallsl6, 18, respectively, and a floor 20. The front end wall 16 has a chargeopening 22 therein and a slidable door 24 is disposedadjacent thisopening. At the upper end of the door 24 is an attaching means 26 whichprovides communication between the door and door actuating means 28which includes a piston 30, pulley 32 and chain 34, said actuating meansbeing operative to lift and lower the door over the opening 22.Dispersed along the floor 20 of the furnace 10 are a plurality of bricks38 which support platform member 40 that receives laterally spaced rowsof longitudinally spaced rollers 42 therein which define rollerconveying means. The platform member 40 has openings 37 at each endthereof. The combination of the openings 37, open brickwork 38 and theplatform 40 yields a gas passageway 41 located beneath the conveyingmeans of the furnace which is part of an overall flue system. workpieces43, or work trays, may be placed upon the rollers and conveyed from oneend of the furnace 10 to the other.

Forming a part of the roof 12 is a drop arch member 46 having a bottomwall 48 and side walls 50. Each of the drop arch side walls 50 has atleast one opening 52 therein and the drop arch 46 segregates the furnaceinto a first chamber 54, a buffer zone 56 located immediately below thedrop arch and a second chamber 58. The first and second chambers havethermocouples 55 and 57 located therein respectively. Received withineach opening of the side walls 50 is a direct fired burner 59. Eachburner 59 has an annular burner housing 60 with a central opening 62therein that defines a combustion chamber. Secured to the outside ofeach side wall 50 and confluent with the combustion chamber 62 is a gasmixing housing 64, there being a gas pipe 66 and an air pipe 68 securedto the housing. Ignition means 70, such as a spark plug, is provided toignite the gas-air mixture within the burner 59.

Immediately adjacent the furnace 10 is an incinerator 72 having annularside walls 73 and a stack 75 extending from the upper end thereof. Thebase 76 of the incinerator has an opening 78 therein which receives aburner 80. Wall means 82 define a passageway 84 that is confluent withthat portion of the passage 41 immediately below the drop arch 46, toform a flue system therewith.

As can be seen by the drawing, the direct fired burners 59 fire directlyinto their respective chambers 54 and 58. As the burners fire into thechambers 54 and 58, their discharging gases entrain the atmosphere gasestherein so that a portion of the atmosphere gases is carried along byentrainment thereby creating a circulation of the furnace atmosphere.The amount of entrainment is determined by the ratio of the length ofthe furnace chamber 54 or 58 to the diameter of the burner 59, eachfurnace chamber having a length L and each burner having a diameter D asseen in FIG. 1. Preferably, the L to D ratio will vary from 20:1 to80:1, It will be observed that the burners are located above the workand fire over the top of the same towards the end walls 16, 18. Thisresults in high entrainment which assures best temperature uniformity ineach chamber 54 and 58.

It has also been found that proper choice of the relative dimension ofthe furnace is helpful in obtaining the desired results, i.e., gastemperature uniformity, high circulation of atmosphere gases, wax vaportransportation and the like. The furnace may be so designed that theratio of the height H of the chambers 54 and 58 to the distance from thearch 46 to the burner center line h is from 4:1 to 10:1. Another pair ofdimensions whose relative sizes can be varied for better results is theratio of the height H to the distance from the chamber ceiling to thebottom of the drop arch a. It has been found advantageous to design thefurnace so that the ratio H/a varies from 3:1 to 8:1.

Beneath the work is the passageway 41 which is confluent with thepassageway 84 leading to the incinerator 72. This combination results ina circular motion being created in the chamber 54 and 58 atmospheres asillustrated by the arrows in FIG. 1. As the gases move within theirrespective chambers 54 and 58 and the circulation motion is set up, thespace 56 immediately under the drop arch 46 becomes a stagnation zonethat segregates chamber 54 from chamber 58. Additionally, a portion ofthe atmosphere, an amount equivalent to the products of combustion fromthe burners 59 plus the wax vapors, is continually discharged throughthe flue system comprising gas passageway 41, passageway 84 andincinerator 72.

The temperatures within the chambers 54 and 58 are maintained atdifferent levels, thermocouples 55 and 57 being provided in each chamberto control these temperatures. When such a furnace is used for thepurpose of dewaxing the curing metal casting molds, the temperature inthe first chamber 54 is only raised to approximately l,000F., the normalrange being between 800l,600F. Preferably, the temperature of the firstchamber 54 is raised sufficiently high to allow the wax to vaporize.Additionally, the burners 59 which fire into chamber 54 are fired with astoichiometric proportion of air and fuel so that no excess air ispresent in the chamber 54 which would tend to ignite or burn the waxvapors. Since the wax vapors are at a relatively high temperature, theyare carried away by the flue system 41 and 84 and are kept from enteringthe second chamber 58 by the balanced counterflow motion of the gases.Within the second chamber 58, the temperature is increased toapproximately 1,800F. so that the curing of the molds may take place.Again, because of the means in which the two systems are keptsegregated, the temperature within chamber 58 will have very littleinfluence on the temperature of chamber 54.

In order to avoid pollution, the effluents from the flue system 41 and84 are incinerated in an incinerator 72. The incinerator 72 is providedwith an air rich burner wherein the wax vapors and other effluents maybe completely reacted before being discharged into the 1 environment.

What is claimed is:

1. A longitudinally extending furnace comprising a housing having aroof, floor, sidewalls and end walls, opening means for charging workthrough one end wall, opening means for discharging work from the otherend wall, means for conveying work from the charge end of the furnace tothe discharge end, said roof having a drop arch located intermediatesaid end walls thereby dividing said furnace into two treating chambers,direct fired burner means located within the upper portions of eachchamber, said direct fired burner means including at least one directfired burner located in'each chamber and received within said drop archand directed to fire longitudinally in the direction of its respectiveend wall, each of said burners having an annular housing defining acombustion chamber, means for supplying combustible gas and air to saidcombustion chamber and means for igniting the gas mixture, gaspassageway means confluent with each of said treating chambers andlocated adjacent said floor, flue means confluent with said passagewaymeans, and the ratio of the furnace treating chamber longitudinal lengthto burner diameter ranging from 20:1 to 80:1.

2. The furnace of claim 1 wherein the ratio of the height of a furnacetreating chamber to the'distance from a burner center line to the top ofsaid treating chamber varies from 3:1 to 10:1.

3. The furnace of claim 2 wherein said flue means is located below saiddrop arch and the ratio of the height of one of said chambers to thedistance from the cham ber top to the arch bottom varies from 3:1 to8:1.

4. A longitudinally extending furnace comprising a housing having aroof, floor, sidewalls and end walls,

opening means for charging work through one end wall, opening means fordischarging work from the other end wall, means for conveying work fromthe charge end of the furnace to the discharge end, wall means locatedintermediate said end walls to divide said furnace into two confluenttreating chambers, direct fired burner means located within each chamberand including at least one direct fired burner having an annular housingdefining a combustion chamber located at each chamber end adjacent saidwall means and being directed to fire towards its respective end wall,gas passageway means confluent with each of said treating chambers, fluemeans confluent with said passageway means and the ratio of a furnacetreating chamber length to the diameter of a burner'ranging between 20:1to 80:1.

5. A furnace for heat treating a plurality of workpieces therein atleast two different heat treating operations comprising:

a roof, floor, sidewalls and end walls defining a furnace enclosure;

opening means for charging said workpiece through one end wall anddischarging said workpiece through the opposite end wall;

said roof having a drop arch in between said end walls to divide saidenclosure into first and second heat treat chambers;

burner means including at least one direct fired burner for each chamberreceived within said drop arch, each burner orientated to discharge itsproducts of combustion towards its respective end wall,

6. The furnace of claim 5 wherein said passage means further includessaid floor having an opening in each chamber adjacent said end wallthereof; a plurality of gas passages beneath said floor in communicationwith said opening at one end, and flue passages adjacent said archcommunicating the opposite ends of said gas passages with saidatmosphere.

7. The furnace ofclaim 6 wherein said passage means further includesincinerator means within said flue means to provide a positive draftwithin said gas passages while incinerating unburnt fumes from saidwork-

1. A longitudinally extending furnace comprising a housing having aroof, floor, sidewalls and end walls, opening means for charging workthrough one end wall, opening means for discharging work from the otherend wall, means for conveying work from the charge end of the furnace tothe discharge end, said roof having a drop arch located intermediatesaid end walls thereby dividing said furnace into two treating chambers,direct fired burner means located within the upper portions of eachchamber, said direct fired burner means including at least one directfired burner located in each chamber and received within said drop archand directed to fire longitudinally in the direction of its respectiveend wall, each Of said burners having an annular housing defining acombustion chamber, means for supplying combustible gas and air to saidcombustion chamber and means for igniting the gas mixture, gaspassageway means confluent with each of said treating chambers andlocated adjacent said floor, flue means confluent with said passagewaymeans, and the ratio of the furnace treating chamber longitudinal lengthto burner diameter ranging from 20:1 to 80:1.
 2. The furnace of claim 1wherein the ratio of the height of a furnace treating chamber to thedistance from a burner center line to the top of said treating chambervaries from 3:1 to 10:1.
 3. The furnace of claim 2 wherein said fluemeans is located below said drop arch and the ratio of the height of oneof said chambers to the distance from the chamber top to the arch bottomvaries from 3:1 to 8:1.
 4. A longitudinally extending furnace comprisinga housing having a roof, floor, sidewalls and end walls, opening meansfor charging work through one end wall, opening means for dischargingwork from the other end wall, means for conveying work from the chargeend of the furnace to the discharge end, wall means located intermediatesaid end walls to divide said furnace into two confluent treatingchambers, direct fired burner means located within each chamber andincluding at least one direct fired burner having an annular housingdefining a combustion chamber located at each chamber end adjacent saidwall means and being directed to fire towards its respective end wall,gas passageway means confluent with each of said treating chambers, fluemeans confluent with said passageway means and the ratio of a furnacetreating chamber length to the diameter of a burner ranging between 20:1to 80:1.
 5. A furnace for heat treating a plurality of workpiecestherein at least two different heat treating operations comprising: aroof, floor, sidewalls and end walls defining a furnace enclosure;opening means for charging said workpiece through one end wall anddischarging said workpiece through the opposite end wall; said roofhaving a drop arch in between said end walls to divide said enclosureinto first and second heat treat chambers; burner means including atleast one direct fired burner for each chamber received within said droparch, each burner orientated to discharge its products of combustiontowards its respective end wall, said burner means effective to entrainfumes emanating from said workpieces within said products of combustion;passage means within said floor and in fluid communication with eachchamber at one end and with atmosphere at the opposite end, said passagemeans effective to a. cause said products of combustion to circulateabout chamber to heat said workpieces primarily by convection, and b. toestablish a stagnation zone underneath said arch to separately maintainand prevent the atmosphere in each chamber from commingling with oneanother.
 6. The furnace of claim 5 wherein said passage means furtherincludes said floor having an opening in each chamber adjacent said endwall thereof; a plurality of gas passages beneath said floor incommunication with said opening at one end, and flue passages adjacentsaid arch communicating the opposite ends of said gas passages with saidatmosphere.
 7. The furnace of claim 6 wherein said passage means furtherincludes incinerator means within said flue means to provide a positivedraft within said gas passages while incinerating unburnt fumes fromsaid workpieces.